A wave of panic rippled through Pakistan's commercial capital, Karachi, today after twin explosions targeting a religious procession and a major hospital killed at least 25 people and injured more than 100.

The chaos started when a bomb, thought to have been planted on a parked motorcycle, ripped through a bus carrying minority Shias to a procession in honour of the revered figure Imam Hussein. At least 12 people were killed and 49 injured, including many women and children.

Two hours later a second explosion occurred outside the emergency ward of the city's Jinnah hospital, where the injured were arriving from the first blast. At least 13 people were killed. A bomb disposal squad later defused a third bomb hidden in a television set in the hospital car park.

The violence, blamed on sectarian extremists, triggered scenes of pandemonium across the city. Television pictures showed emotional Shia mourners beating themselves amid clouds of dust outside the attacked hospital. Three ambulances were destroyed.

Hospital officials rushed to move the dead and injured from Jinnah to other city hospitals. Ambulances milled about in several directions in the streets, while dozens of paramilitary rangers and city police rushed to impose security.

Abdul Sattar Edhi, head of the Edhi Foundation and Pakistan's most famous charity worker, said he narrowly escaped injury at the hospital. Vehicles entering other hospitals were checked for explosives.

There were reports of angry mobs gathering on the streets amid fears of violent protests against the failure of the authorities to prevent the explosions.

Sharmila Farooqi, an adviser to the provincial chief minister, appealed to residents to remain calm. "If they want to help their poor brethren who have died, please help with blood donations, but don't create a law and order situation," she told Dawn News television.

Although the twin blasts had an ostensibly sectarian cause, they could have strong political consequences including the destabilisation of the ruling Pakistan People's party.

The sprawling metropolis of 18 million people has been rocked by instability since a similar attack on a Shia procession on 28 December that killed 45 people and wounded several hundred.

Recriminations over the failure of the security forces to prevent that blast deepened existing tensions between the city's main political players - the PPP, the Muttahida Qaumi Movement and Awami National party.

There have been walkouts from the provincial parliament while the city's slums have seen a series of brutal gangland-style killings carried by criminal groups linked to the political parties.